Not Applicable
1. Field of Invention
This invention relates to an apparatus for use in x-ray residual stress analysis. More specifically, this invention relates to an x-ray residual stress analysis apparatus having a radioisotope for generating x-ray emissions.
2. Description of the Related Art
X-ray residual stress analysis, a subset of x-ray diffraction, is a well-known technique for measuring stress in crystalline materials. A detailed discussion of the technique and the various apparatuses that employ this technique for the non-destructive examination of materials can be found in a text by I. C. Noyan and J. B. Cohen, entitled Residual Stress: Measurements by Diffraction and Interpretation, published by Springer-Verlag in 1987.
All of the x-ray residual stress analysis devices discussed by Noyan and Cohen utilize an x-ray tube composed of an anode and a cathode where electrons emitted by the cathode are accelerated at high velocities into the anode. The interaction of the electrons with the anode produces a continuum of bremsstrahlung photons with energies distributed over a wide energy range and a photon with a specific energy that is characteristic of the anode (target) material. Thus, the energy spectrum of an x-ray tube has a characteristic line superimposed on a bremsstrahlung background. For the energy spectrum to be useful, the bremsstrahlung background must be removed or reduced either electronically or by using mechanical filters.
Typically, the characteristic x-ray energies employed in residual stress vary between 5.4 and 17 keV. Elemental radioisotopes emit photons originating in the nucleus or in the atomic shell surrounding the nucleus having energies within this energy range. However, x-ray energies in excess of 10 keV present special detection problems for silicon-based solid state detectors. As photon energy increases, the stopping power of the silicon in a solid state photodiode array declines. More photons simply pass through the active volume of the silicon undetected. The use of a phosphor material as a scintillator optically coupled to a photodetector permits efficient measurement at higher photon energies. Cesium Iodide(Thallium) (CsI(Tl)) and Gadolinium Oxysulfide (GdO2 S) are two such phosphor materials shown to have high sensitivities over a range of wavelengths extending from approximately 10 keV to approximately 100 keV.
The efficiency of a photodiode array (PDA) to directly detect x rays compared to a phosphor screen coated PDA can be examined using a few simple calculation. In a silicon-based PDA, the number of electron hole pairs per x ray directly produced by a 10 keV x ray is:                                           10000            ⁢                          xe2x80x83                        ⁢            eV                                3.65            ⁢                          xe2x80x83                        ⁢                          eV                              ion                ⁢                                  xe2x80x83                                ⁢                pair                                                    =                  2740          ⁢                      xe2x80x83                    ⁢                                    electron              ⁢                              xe2x80x83                            ⁢              hole              ⁢                              xe2x80x83                            ⁢              pairs                                      x              ⁢                              xe2x80x83                            ⁢              ray                                                          (        1        )            
The results in Equation 1 assume that the total energy of the x ray is absorbed in the active region of the silicon.
According to a paper published in 1991 by Valentine et al., entitled xe2x80x9cCharge Calibration of Systems with CsI(Tl), a Photodiode and a Charge Sensitive Preamplifier,xe2x80x9d in Nuclear Instrumentation Methods, 1991, a CsI(Tl) coated PDA will yield an average of 47,900 electron hole (e. h.) pairs per MeV of photon energy for CsI in the temperature range of xe2x88x9215xc2x0 to 40xc2x0 C. The decay constant for CsI is 6 xcexcsec. This is important only for high x-ray interaction rates within the CsI. For 10 keV x rays interacting with the CsI, this produces:                               47          ,          900          ⁢                      xe2x80x83                    ⁢                                                    e                .                h                .                                  xe2x80x83                                ⁢                pairs                            MeV                        ·            0.010                    ⁢                      xe2x80x83                    ⁢          eV                =                  479          ⁢                      xe2x80x83                    ⁢                                    e              .              h              .                              xe2x80x83                            ⁢              pairs                        interaction                                              (        2        )            
In both Equations 1 and 2, the total amount of charge deposited in the photodiode will be determined by the flux of the x-ray beam. For example, if it is assumed that the event rate within a photodiode is one x ray per second, then the continuous current produced in the photodiode would be:                               2740          ⁢                      xe2x80x83                    ⁢                                                    e                .                h                .                                  xe2x80x83                                ⁢                pairs                                            x                ⁢                                  xe2x80x83                                ⁢                ray                                      ·            1                    ⁢                      xe2x80x83                    ⁢                                                    x                ⁢                                  xe2x80x83                                ⁢                ray                                            sec                .                                      ·            1.6                    xc3x97                      10                          -              19                                ⁢          C                =                  0.00043          ⁢                      xe2x80x83                    ⁢          pA                                    (        3        )            
For the CsI photodetector combination and an assumed interaction rate of one x ray per second, the decay time (6 xcexcsec) of the phosphor is much shorter than the event rate within the photodetector; hence, there are no overlapping pulses. Therefore, each photon generated within the scintillator and interacting with the photodiode will be detected. This yields a peak current per x-ray event of:                               479          ⁢                      xe2x80x83                    ⁢                                                    e                .                                  xe2x80x83                                ⁢                h                .                                  xe2x80x83                                ⁢                pairs                            event                        ·                          xe2x80x83                        ⁢                          1                              6                ⁢                                  xe2x80x83                                ⁢                ms                                      ·            1                    xc3x97                      10            6                    ⁢                      xe2x80x83                    ⁢                                    ms              s                        ·            1.6                    xc3x97                      10                          -              19                                ⁢          C                =                  0.0127          ⁢                      xe2x80x83                    ⁢                      pA            event                                              (        4        )            
For a 5.4 keV x ray, the current produced in a bare and a CsI coated photodetector are estimated to be 0.235 and 6.5 pA/event, respectively.
While systems that utilize phosphor screens attached to a fiber-optic bundle optically coupled to an array of photodetectors are commercially available, it is desirable to employ such a phosphor screen/detector combination with an x-ray residual stress analysis device incorporating an isotropic source.
Other x-ray diffraction apparatuses have been developed for use in x-ray diffraction studies using conventional x-ray tubes. Typical of the art are those devices disclosed in the following U.S. Patents:
U.S. Pat. Nos. 5,125,016 and 4,095,103 both employ a conventional x-ray tube as the emission source for the x rays used in diffraction studies. Both the ""016 and the ""103 patents disclose the use of x-ray tubes in conjunction with one or two position sensitive detectors for use in single or multiple exposure x-ray diffraction studies. However, conventional x-ray tubes are too large to be practical when developing a miniaturized battery powered x-ray residual stress apparatus.
Sources of monoenergetic photons have been used for many years in fluorescence analysis of materials. Typical source designs may be found in an article by K. H. Ansell and E. G. Hall, entitled xe2x80x9cRecent Developments of Low Energy Photon Sources,xe2x80x9d published in the text Applications of Low Energy X and Gamma Rays, edited by Ziegler, and published by Gordon and Breach in 1970 and a publication by F. E. LeVert and E. Helminski, entitled xe2x80x9cLiterature Review and Commercial Source Evaluation of Americium-241,xe2x80x9d ORO-4333-1, 1973. In these cases, the radioisotope may be a direct emitter of x rays (e.g., the electron capture process in Fe-55 leading to the 5.8 keV Mn x ray) or the x rays may be generated indirectly by using a monoenergetic source of photons to excite characteristic x rays in various pure element targets. To be of analytic use, the resulting direct or indirect x-ray radiation must be highly monoenergetic with negligible background contributions.
A paper presented by William S. Toothacker and Luther E. Preuss entitled xe2x80x9cRadioisotopes as Zero Power Sources of X-rays for X-ray Diffraction Analysis,xe2x80x9d published in Nucleonics in Aerospace by the Instrument Society of America, 1968, discussed the use of an isotropic source in x-ray diffraction. For purposes of this application, it is important to distinguish between x-ray diffraction and x-ray residual stress analysis. X-ray diffraction is the study of the structure of crystals and complex molecules through the diffractive properties of these bodies. Toothacker taught the use of a radioisotope as an x-ray source for the study of the structure and composition of matter. Given the definition of x-ray diffraction accepted by those persons skilled in the art, Toothacker does not make obvious the use of a radioisotope as a sealed x-ray source for residual stress analysis.
Residual stress analysis is different from x-ray diffraction in that x-ray diffraction is used to identify the composition of matter while residual stress analysis is used to determine the state of a material. Specifically, residual stress measurements are normally performed at 2xcex8 angles greater than 140 degrees whereas x-ray diffraction pattern measurements are normally performed in the forward reflected region, i.e., at 2xcex8 angles less than 90 degrees. As the 2xcex8 angle increases, the x-ray fluence of the diffracted beam decreases. Therefore, it is necessary for the detector and the emitter to be placed in closer proximity in order to maintain an adequate event rate in the detector. Accordingly, the detectors used for residual stress measurement must be more compact than those typically used for standard or powder x-ray diffraction. Toothacker discussed the difficulties encountered when using a sealed source with a detector positioned at a low back reflection angle. The intensity of the reflected beam measured at low back reflection angles is typically greater than the measured intensity at high back reflection angles. As most residual stress measurements are made at high back reflection angles, it is not obvious from the teaching of Toothacker that an isotropic source would be suitable for the low intensity measurements common in residual stress analysis.
That Toothacker does not make obvious the applicability of an isotropic source in residual stress analysis is further emphasized by the size limitations on detectors inherent in residual stress analysis. In order to obtain measurements at the requisite high back reflection angles, the source and the detector must be placed in closer proximity than in conventional x-ray diffraction studies. Accordingly, it is necessary to minimize the size of the detector, including the active detection area, to achieve the desired proximity. However, limitations in conventional electronics present special problems in developing the compact detectors necessary for efficient residual stress analysis. A similar reduction is desired in the size of the x-ray emitter requiring the use of a smaller source. While an isotropic source presents the possibility of smaller sources, the practical problems associated with the use of isotropic sources have prevented its use. Such problems include the low intensity and the lack of resolution associated with the low intensity, which were noted by Toothacker. Accordingly, in light of the difficulties encountered by Toothacker with a large proportional counter and the inherent difficulty in producing smaller detectors, it is not obvious that a position sensitive detector having a smaller active detection area would be effective.
Because of the differences between x-ray diffraction analysis and residual stress analysis, Toothacker does not make obvious to one of ordinary skill in the area of residual stress analysis that a sealed source would have either the intensity required to yield the resolution needed for precise determination of the angular location of the peak of a diffracted x-ray beam or the intensity required for the measurement of residual stress in a specimen.
Accordingly, there is a need for an x-ray residual stress analysis apparatus employing an x-ray source having a size smaller than that which is possible using conventional x-ray tubes. Additionally, there is a need for an x-ray residual stress analysis apparatus having a source which does not generate excessive background noise to the measurements. Further, there is a need for an x-ray residual stress analysis apparatus which is capable of simultaneous multiple angular exposures to reduce measurement times. Still further, there is a need for an x-ray residual stress analysis apparatus which is capable of efficient residual stress analysis when the intensity of the primary and reflected beams from an x-ray source and a target material, respectively, are low. Finally, there is a need for a practical x-ray residual stress analysis apparatus employing an area detector.
Therefore, it is an object of the present invention to provide an x-ray residual stress analysis apparatus which employs an x-ray source comprising a radioisotope that can be selectively exposed to a target material.
Another object of the present invention is to provide an x-ray residual stress analysis apparatus having an x-ray source producing essentially a monoenergetic x-ray beam.
Yet another object of the present invention is to provide an x-ray residual stress analysis apparatus capable of taking simultaneous exposures covering multiple angles.
An additional object of the present invention is to provide an x-ray residual stress analysis apparatus having a detector which is efficient at photon energies greater than 10 keV.
A further object of the present invention is to provide an x-ray residual stress analysis apparatus that does not require an external power source allowing it to be used in remote locations.
A radioisotope based x-ray residual stress analysis apparatus using a shielded, monoenergetic radioisotope, or isotropic, source to emit x rays for measurement of the stress state of a polycrystalline material. The isotropic source is selected from spontaneously emissive radioisotopes emitting photons in the five to one hundred (5-100) keV energy range. The source assembly may be sealed or unsealed. The source may be a direct emitter of monoenergetic x rays or it may be selected from a class of indirect x-ray emitters where a source of radiation is used to produce the characteristic x ray of an integral target material.
The isotropic emissions are focused into a beam directed at a point on the sample. The sample diffracts the emitted beam into a diffraction cone, represented by diffracted beams. The diffracted beams are detected by at least one position sensitive detector, either directly or indirectly.
The source assembly and position sensitive detector are connected in a manner that allows mechanical variation of the angle, "xgr", between the two position sensitive detectors or the angle, xcex2, between the source assembly and the sample perpendicular. When the diffracted beam strikes the position sensitive detector, an electrical charge proportional to the x-ray energy absorbed in each of the photodiodes is created and stored therein. This charge is subsequently conditioned and outputted as a voltage signal corresponding to a position within the photodiode array. The position sensitive detectors are electrically coupled to conventional signal conditioning electronics which amplify and otherwise condition the signal. The conditioned signal is transferred to an analysis and storage device.
As a result of the minimal shielding required for the source and the small size of the isotropic source, the x-ray residual stress analysis apparatus of the present invention is uniquely suited to be configured with an area detector.
The position sensitive photodiode array(s) may be replaced with gas filled position sensitive proportional counters (PSPCs). In addition to normal residual stress analysis, the use of a PSPC allows the identification of characteristic photons emitted by particular isotopes through the discrimination of output pulses having an amplitude proportional to the energy of incident photons. When conditioned signals from the PSPC are analyzed by an analysis unit, these secondary x-ray fluorescence (XRF) photons can be used to identify trace elements within a sample.
Finally, the present invention is designed to be powered by commercially available dc batteries allowing residual stress analysis to be performed in remote locations, such as bridges and deserts, where other sources of electronic power are not readily available.
The present invention offers the capability to make non-destructive measurements of the stress state of machinery actually operating in the field under normal operating conditions. One such area where frequent field measurements are desirable is the railroad industry. The rails of railroad tracks and the wheels of railroad cars are exposed to high stresses due to increased traffic, speed, and axle loads. Using the portable, radioisotope-based residual stress analysis device of the present invention, it is possible to go to remote locations and rapidly measure the in situ longitudinal stresses in rails and the residual compressive stresses in wheels.